With each successive semiconductor technology generation, wafer diameters tend to increase and transistor sizes decrease, resulting in the need for an ever higher degree of accuracy and repeatability in wafer processing. Semiconductor substrate materials, such as silicon wafers, are processed by techniques which include the use of vacuum chambers. These techniques include non plasma applications such as electron beam evaporation, as well as plasma applications, such as sputter deposition, plasma-enhanced chemical vapor deposition (PECVD), resist strip, and plasma etch.
Plasma processing systems available today are among those semiconductor fabrication tools which are subject to an increasing need for improved accuracy, repeatability and efficiency. Success metrics for a plasma processing system include throughput and substrate temperature stability. Substrate temperature affects critical dimensions of devices fabricated on a substrate and thus must not significantly drift when stable substrate temperature is required, e.g. within a step in a processing recipe.
For example, poly-silicon gate etching is driving towards smaller and smaller critical dimension uniformity (CDU) to be achieved across a substrate of about 300 mm in diameter. Such a variation could be due to radial variation in substrate temperature near the edge, plasma chemistry or density, an overhanging edge ring, or other constraints. The CDU requirements are expected to become more stringent with the continuing reduction in node size.
Fabrication of an integrated circuit chip typically begins with a thin, polished slice of high-purity, single crystal semiconductor material substrate (such as silicon or germanium). Each substrate is subjected to a sequence of physical and chemical processing steps that form the various circuit structures on the substrate. During the fabrication process, various types of thin films may be deposited on the substrate using various techniques such as thermal oxidation to produce silicon dioxide films, chemical vapor deposition to produce silicon, silicon dioxide, and silicon nitride films, and sputtering or other techniques to produce other metal films.
After depositing a film on the semiconductor substrate, the unique electrical properties of semiconductors are produced by substituting selected impurities into the semiconductor crystal lattice using a process called doping. The doped silicon substrate may then be uniformly coated with a thin layer of photosensitive, or radiation sensitive material, called a “resist”. Small geometric patterns defining the electron paths in the circuit may then be transferred onto the resist using a process known as lithography. During the lithographic process, the integrated circuit pattern may be drawn on a glass plate called a “mask” and then optically reduced, projected, and transferred onto the photosensitive coating.
The lithographed resist pattern is then transferred onto the underlying crystalline surface of the substrate through a process known as etching. A plasma process chamber is generally used for etching substrates by supplying a process gas to the plasma process chamber and application of a radio frequency (RF) field to the process gas to energize the process gas into a plasma state.